1. Field of the Invention
This invention relates to chemical landfills; more particularly to the disposal of contaminated wastewater sludges in a closable impoundment.
2. Description of the Prior Art
Growing public concern over pollution of the earth's water supply by industrial and municipal liquid waste has caused considerable attention to be focused on the proper treatment of wastewater before permitting it to come in contact with natural water sources. "Wastewater" as used herein means any liquid which contains an aqueous component together with contaminants either dissolved or suspended therein. Thus wastewater would include, among others, aqueous effluents from industrial activities and municipal sewage.
The typical wastewater treatment process involves the collection of dissolved or suspended contaminants and then the disposal of these collected contaminants which are commonly referred to as "sludge." Sludge, more precisely, comprises a mixture of solids (both insoluble and soluble) and water. Traditionally, disposal of sludge has been accomplished either by burying it in the ground or by incinerating it. The present invention is concerned only with the former, land disposal.
Various proposals have been made over the years for the land disposal of wastewater sludges but each has been unacceptable from the standpoint of modern water pollution standards. More specifically, the proposed disposal techniques either caused direct pollution of surface water or groundwater, or involved ultimate sludge disposition that indirectly resulted in such pollution, the latter occurring by virtue of dissolved contaminants in the water portion of the sludge mixture. Such contaminated solutions are unacceptable wastes in themselves and are capable of pollution if discharged to groundwater or surface water without proper treatment.
Exemplary of the problems involved in sludge disposal is the difficulty experienced in permanently and safely disposing of spent industrial acid solutions. Industrial acids are used in a variety of applications; a principal application is in the process of chemically removing oxides and scale from metals called "pickling." In the iron and steel industry, for example, pickling with dilute sulfuric and hydrochloric acid solutions finds wide use in the manufacture of sheet and tin plate products. In the pickling process, metal oxides on the metal surface being treated combine with the acid to form metallic salts which dissolve in the acid solution; as the concentration of metal salt increases, the acid solution loses its effectiveness and is considered "spent."
Some attempts have been made at regeneration of spent pickle liquors. However, because of the relatively low cost of acid, many industrial organizations prefer to dispose of the spent pickle liquor and have tried various alternatives. Neutralization of the acid material to precipatate insoluble reaction products is a widely-used initial step in pickle liquor disposal; this step is followed by some type of disposition of the precipitated sludge. Another disposal technique is to first remove the larger solids from the spent acid and then pump it into deep underground wells. This latter method is expensive and available only where geological conditions permit it.
The neutralization/sludge disposal technique for spent pickle liquor is confronted with a variety of problems. The process usually consists of mixing the waste liquor with lime or other alkaline agent to form metal hydroxides; this sludge-like material is then discharged into lagoons and permitted to settle. The supernatant liquid may or may not be removed and treated depending upon the circumstances. However, no attempt is made to dewater the sludge itself; as a result, the metal hydroxide sludge remains in a fluid, unstable condition in the lagoon; therefore, the lagoon cannot be covered. Further, conventional lagooning methods often ignore the fact that contaminated water, present either with the sludge mixture or resulting from rainfall, may migrate through the base of the lagoon to pollute subsurface ground water. This contaminated water, often called leachate, can be produced by chemical reversion in the neutralized sludge and will always pose a hazard when an open sludge lagoon is located in an area where rainfall exceeds the evaporation rate.
To overcome the problem of uncontrolled migration of leachate from an open lagoon, some sludge disposal systems have been constructed to make the disposal bed or lagoon impervious to liquid penetration. This technique prevents pollution of groundwater or surface waters through leachate migration; it does not, however, solve the problem of closure of the lagoon and, if anything, aggravates it because the watertight basin retains the sludge solids and water solution as a fluid, unstable mass.